[MLIR][SPIRV] Added optional name to SPIR-V module

This patch adds an optional name to SPIR-V module.
This will help with lowering from GPU dialect (so that we
can pass the kernel module name) and will be more naturally
aligned with `GPUModuleOp`/`ModuleOp`.

Reviewed By: mravishankar

Differential Revision: https://reviews.llvm.org/D86386